1. Field of the Invention
This invention relates to an analog switching circuit, and more particularly to an analog switching circuit using an insulated-gate type field-effect transistor (referred to hereunder as MOST).
2. Description of the Prior Art
Analog switching circuits are widely used in various fields of applications such as an analog multiplexer, a sample and held circuit (referred to hereunder as S/H circuit), and a switched capacitor filter. Since frequency band and dynamic range of the signals treated by the application circuits have been expanded, a high-speed operation, low noise and high precision are required for the analog switches. Further, the above-mentioned application circuits are usually formed as integrated circuits (referred to hereunder as IC), so that the analog switches are required to be readily formed as a part of IC. The analog switch usually employs a transfer gate consisting of MOST as a switching means. More specifically, one and the other of source and drain of the MOST are used as input and output terminals of analog signals, respectively, and switching pulses are applied to the gate of the MOST thereby to close or open the MOST for carrying out switching for the analog signals. Since such analog switch inevitably has a stray capacitance between the source-drain and the gate of the switching MOST, a switching time includes a delay and the transient noise is generated at the output terminal on switching operation.
In order to cancel the transient noise at the output terminal, a capacitor is connected to the output terminal at its one terminal and pulses in opposite phase with respect to the phase of the switching pulses are applied to the other terminal of the capacitor. Usually, the compensation capacitor comprises a MOST having half the size of the switching MOST. The capacitance between source-drain and gate of the compensation MOST is utilized to cancel the transient noise at the output terminal. However, it is difficult in usual IC-manufacturing processes to control precisely the size ratio between transistors having considerably different sizes, so that, when the analog switching circuit with the compensation MOST is integrated into IC, the transient noise can not be completely cancelled, with the result of decreased precision of signals. In addition, the switching MOST taken an undesirably large size to increase the switching time and narrow the frequency band which can be treated.
It is therefore the object of this invention to provide an analog switching circuit which can operate with a high speed and little transient noise and can be easily formed in an IC.